1. Field of Invention
The field of the currently claimed embodiments of this invention relates to photovoltaic devices, and more particularly to metal-chalcogenide photovoltaic devices that have a metal-oxide nanoparticle window.
2. Discussion of Related Art
Many metal-chalcogenide semiconductors are being used for thin film photovoltaic devices due to high photon absorption and high efficiency relative to other thin film photovoltaic materials, such as amorphous silicon. In one configuration, soda lime glass is used as a substrate with a layer of molybdenum deposited on it. A metal-chalcogenide semiconductor, such as CIGS for example, is then formed on the molybdenum layer. This provides a p-type semiconductor layer upon which a second metal-chalcogenide layer of an n-type material is formed. It is believed that sodium in the soda lime glass provides beneficial properties. In other cases, other materials are used for the substrate, such as flexible materials, and sodium can be included by other means.
The molybdenum layer serves as an electrode as well as reflecting light back into the photon absorption layer of the metal-chalcogenide. A window layer is formed on the metal-chalcogenide absorption layer. The window layer can be consider to serve as, or to include a transparent electrode. Alternatively, one can consider that there is a transparent electrode formed on the window layer. The metal-chalcogenide photon-absorption layer is generally a p-type semiconductor. An n-type semiconductor buffer layer is formed on the p-type layer. Conventional devices use a buffer layer of CdS on the metal-chalcogenide photon-absorption layer. It is desirable for the window layer to be transparent to light within the region of the electromagnetic spectrum that is strongly absorbed by the metal-chalcogenide semiconductor layer. A layer of intrinsic ZnO (i-ZnO) is provided to protect the device during further processing, and then a layer of Al doped ZnO (ZnO:Al) is sputtered onto the device for the window layer. Further layers are often added, including transparent electrodes.
This process of including many layers as well as sputtering is a complex and expensive manufacturing process and the sputtering can damage the underlying structure. Furthermore, conventional devices also use a significant amount of cadmium, which is toxic. Therefore, there remains a need for improved metal-chalcogenide photovoltaic devices and methods of manufacture.